TITANIUM 99: SCIENCE AND TECHNOLOGY. THE EFFECT OF Ji-STA_BIL~ZING ELEMEN':fS ON THE. TENSILE STRAIN BEHAVIOR OF SINGLE

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1 TITANIUM 99: SCIENCE AND TECHNOLOGY THE EFFECT OF Ji-STA_BIL~ZING ELEMEN':fS ON THE. TENSILE STRAIN BEHAVIOR OF SINGLE '. '.. '. a-type Ti-ALLOYS AT 20K* Dr; YANG GUANJUN, Prof. CAI XUEZHANG,.Eng. DU YU, En~.-zHANG WEIFENG~ Prof. DENG JU, Pror ZHOU LIAN Northwest Institute.for Nonferrous Metal Research r.'o. B~x51, Xi'an, Shaanxi , P. R. China. ABSTRACT In the present work, a single a -phase al,loy and a near a -phase alloy with a little amount of P~stabilizing elements, the microstructure of them. is very different,. ~ere selected fodnvestigation on tensile 'strain behavior at 300K and 20K. The tensile tested results showed that in both alloys, the tensile curves at 20K became serrated, and the number of s erration corresponds with the number of necking in the specimen gage section. While in the ~... microstructure.oftensile strained at 20K, there.were a large number of_deformation twins.in the singl~ a -phase... ~.. alloy,. but. in the near- a -phase alloy, it is hardly to find deformation twins.. These results suggest that the serration of the tensile curve is not directly related with the deformation twins appeared in the tensile strained microstructure, and is real deal with the necking in the specimen gage section'. Some reasons p~obably caused multiple necking are discussed in the paper. Key words: Titanium alloy, cryogenic properties, strain behavior I. INTRODUCTION Having a higher stren~th and ductility at cryogenic temperature, some Ti~alloys ar~ impo~t candidate for cryogenic engineering. The cryogenic properties of single a-ti alloys are mc;>re excellent, e.g. Ti-5Al-2.5Sn (ELI), could be used at the lowest temperature to 4.2K[ 1 l. The said alloy have been used for manufacturing of pressure cantainer of liquid Hydrogen in space vehicles[ 21 Alloying elements, microstructure', stram_temperature and strain rate, all t.hese factors c_old~ affect strain behavi.or at cryogenic temperature[ 3 41 Hot workability of the a-ti alloy containing Al, Zr or Sn etc elements usually isn't well such as that of the near a-ti alloy containing a small amount of P-stable elements e;g. Nb, V, Mo etc. Many investigations on cryo_genic strain-behavior of Ti Al-Sn (Zr) single a-ti alloys have been reported[ , but in~estigation on cryogenic strain behavio~of_near a-ti alloy is lacking. In the present work, a single a-ti alloy containing Sn or Zr and a near a-ti alloy containing a small amount of P-stable elements have been used for testing and studying of the tensile properties and deformation mechanism at 20K. 2. MATERIALS AND EXPERIMENTAL PROCEDURES The ingots of Ti-alloys; their composition were previously designed according to. requisite properties o( strength and ductility, were prepared by doubly-var melting, and then hot-forged and hot- and cold-rolling into 1.5mm I I. ' '.. ' * Supported by the National Advanced Materials Committee of China (NAMCC) 616

2 TITANIUM. 99 SCIENCE AND TECHNOLOGY thickness sh~ets. The sheets were annealed at 800 C for I hour, and cooled in air. Specimens with gage of 15mm width and 25mm length for tensile test at 300K and 3mm width and 15mm length for at 20K and 77K. The tensile-strained specimens were metallographically prepared for observation of the strained microstructure and analysis of strain fracture mechanism. The content of impurities in the alloys is listed in Table l. Table I Impurity content of the alloys alloys normal composition F CTI Ti-3Al+Ma 0.04 impurity content wt% C N 0 H CT3 Ti-3Al+Ma + M~ 0.04 Note: M"=a-stable elements, e.g. Sn or Zr M 0 =P-stable elements, e.g. Nb, V or Mo RESULTS AND DISCUSSION 3.1. MICROSTRUCTURE C "\ The microstructure of the annealed sheets can be see.n in Fig. 2. The equiaxed crystal with about IOµm grain size..,. ' of CTI alloy sheet after annealing manifest a completely r_ecrystalized microstructure, see Fig. 2(b). The microstructur.e of the CT3 alloy annealed sheet is consisted of small a equiaxed-grains with about 2-5µm grain,..., size and the fine P-particles are distributed along the.a grain boundaries, see Fig. 2(d) TENSILE PROPERTIES The tensile propertie~ of CTI and CT3 alloys at 300K, 77K.and 20K are listed in Table 2. In comparison of the tensile properties at 300K, the tensile strength of CT3 alloy increased 9bviously as adding P-stable elements, but its elongation decreased. Under cryogenic temperature, for both CTI and CT3 alloys, strength values grow up almost in 2-fold of that at 300K, elongation values fall down to about 10%.. Table 2 The tensile properties of CTI and CT3 alloy sheets temperature direction properties.. al.loys. 0 CTI en UTS (MPa) T Ys (MPa) " k. EL (%) UTS (MPa) 584,728 C L YS (MPa) EL (%) K L UTS (MPa) EL (%) K L UTS (MPa) EL (%)

3 TITANIUM. 99: SCIENCE AND TECHNOLOGY 3.3. TENSILE STRESS-STRAIN BEHAVIOR AND STRAINED MICROSTRUCTURE (a) 20K (b) 20K 'O (IJ 0...J Displacement Displacement Fig. I Tensile load-displacement curves of CTI (a )and CT3 (b). The tensile load-displacement curves of both alloys are shown in Fig. I. At 300K and 77K, the curves of both CT I and CT3 appear a continuously strain-hardening fashion. and at 20K, the curves of both alloys become serrated. Observation of the surface of the tensile strain-fractured specimens at 300K showed that slip-stain appearance was over all grains in the gage of the specimen. That means slip-strains occur popularly and continuously in grains of the specimen gages and finally necking appeared until fracturing. While at 20K, the multiple shear strain bands or neckings appeared discontinuously on the gage surface and finally fracturing happened at some. one among them. That means strain is locally and disconnectedly, and takes placed preferentially at some positions of the gage. Multi-strain bands or multi-neckings are similar to the results of the works1 4 l, it is considered that multiple neckings are correspondingly related with serrations on the curves of tensile specimens. Fig. 2 The strained microstructure of CTI and CT3 alloy sheets tensile tested at 300K a)ct I, necking zone b)cti, non-necking zone c)ct3, necking zone d)ct3, non-necking zone 618

4 TITANIUM. 99 SCIENCE AND TECHNOLOGY Fig. 2 shows the strained microstructure near the fracture and non-necking zone in the tensile specimen gage. At 300K, the strained grains elongated along tensile-stress direction in both CT I and CT3 alloy specimens, the slipstrain striations and grain twisting deformation and which resulting in cracking along grain boundaries or crossing sites of the strained striations could be observed, see Fig. 2 {a, c). These observations suggest the slip strains dominate the deformation of CTI and CT3 sheets tensile tested at 300K and 77K. Fig. 3 shows that the strained microstructure of CTI and CT3 sheets at 20K. A lot of deformation twins appeared in the strained microstructure of CTI, see Fig. 3 (a, b), that means a deformation mechanism combinating of slip strain and deformation twins dominate the tensile strain behavior of CT I at 20K. As for CT3, the tensile deformation is still dominated by the slip strain, because it is hard to find any deformation twins in the strained microstructure, see Fig. 3(c, d). Fig. 3 The strained microstructure of CT I and CT3 sheets tensile tested at 20K a)cti, necking zone b)cti, non-necking zone c)ct3, necking zone d)ct3, non-necking zone 3.4. DISCUSSION ON DEFORMATION MECHANISM AT CRYOGENIC TEMPERATURE At 20K, the tensile curves of CT I and CT3 become serrated, it is a discontinuously strain-hardening behavior different from that of tensile curves 300K and 77K. The serrations on the tensile curves are correspondingly related with multiple neckings or shearing bands in the gage of tensile specimens. Essentially the serrations on the curves and the multiple neckings or shearing bands in the gage manifest a discontinuously and locally straining behavior. The localization and discontinuation of tensile straining at 20K is caused by properties inherent in material. The investigation of N.V.Ageev et a11' 1, on yielding strain mechanism of a single crystal of Ti-Al-Sn a-ti alloy indicated that the shear stresses for prismatic slip have a strong negative temperature dependence, while the shear stresses during twinning have a weaker dependence on temperature. It suggests that the yielding stresses for slip strain of polycrystal would be strongly raised with decreasing of temperature, the shear stresses of twinning would be less changed. Based on the above reasons. at 20K, the yielding stresses of 619

5 TITANIUM 99 SCIENCE AND TECHNOLOGY CT I alloy slip strain would be remarkably raised, -while the shear stresses of CT I alloy twinning wouli:l be less varied, and thus its slip strains become difficulty, under higher stresses, the twinning become easily acting, thus tensile strain could be accompanied by a lot of deformation twins. The deformation twinning during tensile strain can relax the internal stresses caused by local deformation and promote new slip straii1 or twinning. On the workr 4 1, the discontinuously' and locally strain behavior is explained with adiabatic deformation which caused a plastic instability at cryogenic temperature. The thermal feed back during the adiabatic deformation advances continuously deformation along the direction of the largest shear stresses. But this deformation zone appears as narrow band and don't broad to a larger area due to strain-hardening and stop 'of the thermal feed back. 111" comparing with grain strain degree at 300K, deformation degree of the grain strained at 20K is quite small, see Fig. 3, because the slip strain is difficulty and the small contribution to deformation of twinning itself determine the less deformation degree of grains. As for CT3 alloy, although the serrations and the multiple neckings appeared, their number is less than that of CTI alloy, it is har?iy twinning and only de~end on difficult slip strain and thus its elongation is less than that of CTI alloy. In the light of above results we can come to following conclusion: the serrations on the tensile curves are directly related to the macro-shear bands or neckings in the gage of tensile specimens: Factors affecting on the strain behavior of mulr iple necking or shear bands dealing with serration of tensile curves include alloy composition, microstructure, strain temperature, strain rate and even include geometry and size of specim~ns. But key points are alloy composition, microstructure, strain temperature and strain rate. Main reasons resulting in the difference between CTI and CT3 strain behavior are their alioy composition and microstructure. Based on the workr 4 1, under the condition of cryogenic temperature, the deformation behavior of Ti-alloy also is more sensitive to strain rate, with a higher or a-lower strain rate, the number of multiple neckings during tensile strain will be decrease and even didn't happened. The alloy composition and the microstructure favorable to twinning can caused ~uch more serrations on the tensile curves and get a larger elongation, otherwise will reduce or stop serration strain and get a less elongation. Therefor, in order to get a good match of strength and elongation of a Ti-alloy, a carefully consideration must be given to select alloy.composition and m icrostructure. The deformation rate of twinning is faster than that of dislocation slip strain especially at cryogenic temperatures, so the occurr~nce oftwi;ming could promote popularly development of the multiple neckings or shear bands, and occurring of more serrations on the c urves. Therefor considering of Ti~alloy composition and microstructure, alloying elements and microstructui-al types favorable to twinning should be preferentially selected CONCLUSION. I )CT I all~y with single a-phase has good ductility at 300K and,20k. CT3-Ti alloy containing.a small amount.of P-.stable elements could raise strength and improve hot work ability, and still retain good mechimical properties at cryogenic temperature. 2 )The slip strains dominate the tensile deformation at 300K for both alloys; while at 20K, the deformation i mechanism is c?mbination of slip strain and t)ninning for CT I alloy, and slip strain f~r CT3 alloy. 3)At 20K, serrations on tensile curves and multiple neckings or shear bands on gage surface of tensile specimens appearfor both alloys, but the number of serrations and multing neckings of CTI alloy is more than that ofct3 alloy. The composition and microstructure of both alloys mainly cause this difference. 620

6 TITANIUM. 99: SCIENCE AND TECHNOLOGY REFERENCE. I. E. A. Borysova, S. E. Belyaev, G. S. Klimova: "Properties of Ti-alloys at Cryogenic Temperature", Metals, 1968, No. 4, P (In Russian). 2. Materials in Design Section Data: "Properties of Alloys for Low Temperature and Cryogenic Service", Metal Progress, 1969, Vol. 96, No. I, P Hisaoki sasono and Hirozo Kimura: "The Effect of Deformation Twinning on Mechanical Properties of a-ti.alloys at Low Temperatures", Nippon Metal Sociaty Journal, Vol 41. P Takeshi Kawabata, shigetaka Morita and Usamu Izumi: "Deformation and Fracture ofti-5al~2.5sn El,,I Alloy at 4.2K-29 I K", Ti'80 Science and technology, Vol , P N. V. Ageev, E. 8. Rubina, A. A. Babarekot etal: "The Influence of Deformation Mechanism on Yielding Characteristics in a Single Crystal of Ti-Al-Sn a-alloy", Ti'80 Science and Technology, 1980, Vol 2, P CAI Xuezhang, YANG Guan jun, DU Yu, DENG Ju and ZHOU Lian: "Stress-strain and Fracture of Ti~3Al- 2:5Zr Alloy at Cryogenic Temperture''., The proceedings of 1998 Xi'.an International Conference on Titanium, edit~d by ZH.OU Lian etal. Pub._ 1999, Xi'an China, P